1. Field of the Invention
The present invention relates to a field emission type electron gun useful for e.g. electron beam exposure devices or electron beam devices for high brightness.
2. Description of the Prior Art
A typical conventional field emission type electron gun is illustrated in FIG. 1, wherein a Butler's anode is employed as the anode. A filament 4 for heating a first anode 2 to generate a gas is disposed obliquely above the first anode 2 in order to avoid a disturbance of the electric field which is likely to take place if the filament is disposed immediately above the first anode 2. In the Figure, reference numeral 1 designates a cathode, numeral 3 designates a second anode, numeral 5 designates a vacuum tube, numeral 6 designates a power source for heating the filament 4, and numeral 7 designates a power source for emission of electrons.
In such an electron gun, the first anode 2 is heated by the filament 4 (which is heated at a temperature of about 2500.degree. C.) disposed obliquely thereabove, and the temperature of the surface of the first anode 2 will be as low as some hundreds degrees in centigrade, whereby e.g. oxygen atoms having strong bond strength will be retained on the surface of the first anode 2 while adsorbed molecules (e.g. water) having a small bonding energy can be removed. Accordingly, with an increase of the emission current, there will be an increase in the probability of the liberation or ionization of the adsorbed gas caused by excitation of electrons. The liberated molecules, atoms or ions are likely to be adsorbed or impinge on the surface of the cathode 1, and thus tend to change the work function of the electron emitting surface. Thus, there has been a difficulty that the emission current tends to be unstable.